A number of sensors for detecting an operative state of each location in an installation such as press, machine tool, construction machine, ship, aircraft, unmanned conveyor, unmanned warehouse or the like as well as a number of actuators for controlling an operative state of each location in the above-described installation are required to centrally control the installation. For example, in the case of a press, more than 3,000 sensors and actuators are required. With respect to other installations, much more sensors and actuators will be required.
Hitherto, a centralized control system for centrally control this kind of installation is constructed such that a number of sensors and a number of actuators are connected to a main controller which receives outputs from the sensors and then controls the actuators in response to signals from the main controller.
With such conventional centralized control system, when the number of sensors and the number of actuators become very large, the result is that the number of conductors extending between the main controller and the sensors/actuators becomes very large and input/output sections in the main controller become very complicated in structure.
In view of the aforementioned fact, there has been made such a proposal that a plurality of nodes are connected in series one after another, one or plural sensors and one or plural actuators are connected to each node and these nodes are annularly connected to each other via a main controller to control each node in response to a signal from the main controller. In the case of this proposal, basically, the main controller requires only signal input conductors and signal output conductors and also each node requires only signal input conductors and signal output conductors. Consequently, the number of required conductors can be reduced remarkably.
However, in the case where the nodes are connected to each other in series in the above-described manner, there arises a problem in respect of how concurrence of receiving an output from each sensor and concurrence of controlling each actuator are assured for each node. For example, in the case where an address is allocated to each node which in turn is controlled based on this address, there arises another problem that time delay occurs due to a necessity for address processing. Therefore, the aforementioned proposal fails to assure satisfactory concurrence in relation to receiving of an output from each sensor and controlling of each actuator.
For the reason, the inventors have abandoned the above-described technical concept that a plurality of nodes are connected to each other in series and a certain address is allocated to each node. Instead of this technical concept, they have proposed a new approach to an apparatus for carrying out serial control wherein the respective nodes are distinguished from each other in accordance with an order of connecting the nodes to each other with the result that any address processing is not required, time delay accompanied by such address processing does not occur and each node can substantially be simplified in structure.
According to this proposal, ,each node is constructed such that a signal from each actuator is successively added to a signal from the node at the preceding stage in accordance with a predetermined rule and then a predetermined signal is successively subtracted from the signal from the node at the preceding stage in accordance with another predetermined rule to output the resultant signal to another actuator. In this case, no address is required for each node and thereby no address processing is required. Consequently, time delay appearing at each node can be minimized to such an extent that timing only is controlled correctly. Moreover, each node can substantially be simplified in structure.
In the case of the aforementioned proposal, the respective nodes and the main controller make a determination with reference to an order of signals (positions assumed by data among a number of data) as to from which node a certain signal (data) is received or to which node the signal (data) should be sent. However, if an error is caused by addition or removal of a signal at each node, the main controller can not make a determination as to from which node the signal is received and to which node it should be sent. Consequently, the apparatus becomes inoperative in control. Sometimes, there is a danger that the apparatus runs recklessly.
As will be apparent from the, above description, with the conventional apparatus, when an error is caused by addition or removal of a signal, i.e., when incorrect bit addition or incorrect bit removal occurs, the resultant incorrect signal is transmitted to the node at the subsequent stage and the main controller, whereby the node at the subsequent stage and the main controller do not operate correctly. In an extreme case, the apparatus may run recklessly. Such a problem becomes remarkable particularly in a case where any address is not allocated to each node but the respective nodes are distinguished from each other in accordance with an order of connection of the nodes.
Thus, the present invention has been made with the foregoing background in mind and its object resides in providing an apparatus for carrying out serial control which assures that a data row length abnormality due to incorrect bit removal or bit addition at each node can be detected reliably and incorrect operation and reckless running of an associated apparatus or installation to be controlled can be prevented without fail.